An insole is an element inserted between a shoe and the foot to reduce local pressures in sensitive spots on the plantar surface of the foot so as to prevent or reduce pain and future damage to the foot and to the locomotor system. This is accomplished by fixing the foot in a certain position and orientation relative to the shoe, and by controlling the shape of the insole/foot interfacial surface.
Several types of insoles are widely marketed. For certain problems, a standard, pre-cut insole which can be mass produced in various shoe sizes and fit into conventional shoes suffices. However, individually tailored shoe inserts which are shaped to the individual foot are always preferable, especially in certain foot disorders. Such inserts are generally made by a process which is both laborious and time consuming. A plaster cast is taken of the affected foot and, using this cast, an insert of the proper size and shape is built by hand and fit into the shoe.
Other methods disclosed in the literature are less time consuming than this method being based on molding an insole inside the shoe. The first such method is a method of producing inserts for ordinary shop-bought or standard "deep shoes", disclosed by R. G. S. Platts, S. Knight and I. Jakins in an article entitled "Shoe inserts for small deformed feet", Prosthetics and Orthotics International, 1982, Vol. 6, pp. 108-110. This method involves molding the insert in the shoe either using the foot itself or a positive cast of the foot. The method includes preparing a shoe-shaped "polythene" bag, which is cut, sealed and heat shrunk onto a last of approximately appropriate size for the shoe, and preparing an insole base for stiffening. The insole base is placed in the polythene shoe bag and both are placed in the shoe. If using the patient's foot directly, the foot is clothed in stockinette and covered with a shaped sock made of Ambla P072 having a polyurethane film which is painted with a release agent where adhesion to the foam is not required. If using a cast, the cast is covered with a thin latex sheet.
The components of a flexible self-generating polyurethane foam are mixed and quickly poured into the polythene bag in the shoe. The foot or cast is placed in the shoe and the correct attitude is maintained for a further two minutes until the foam hardens. The patient should bear weight on the foot.
Once the foam has hardened, the foot and the polythene bag are removed from the shoe and the insert is trimmed as necessary. A layer of preformed polyurethane foam is added to complete the insert.
It is a disadvantage of this method that the uncontrolled pressure created within the shoe during molding causes a change in foot shape and placement relative to the shoe. The method is also relatively laborious and time consuming.
A second method of preparing an insole inside the shoe is disclosed in U.S. Pat. No. 3,895,405. This method comprises placing a flexible foam insole member into a shoe, heating the insole to a temperature sufficient to cause the foam to lose some of its resiliency, placing a foot in the shoe before the insole regains its resiliency, and taking steps with the foot in the shoe until the insole regains its resiliency.
This method suffers from the difficulty of obtaining the desired height of the insole by the molding process itself, thus requiring additional steps of trimming or adding layers to the insole, which lengthen the process of insole production. Furthermore, thermoplastic foam materials tend to deform and lose their original shape after a short period of time.
U.S. Pat. No. 4,128,951 to Tansill describes an insole which includes outer and inner containers. The inner container holds a quantity of liquid catalyst, while the remainder of the interior of the outer container is filled with a curable liquid elastomeric material. In the technique of insole configuration, the inner container is rendered frangible and then the outer container is compressed to rupture the inner container, producing contact between the catalyst and the curable liquid elastomeric material.
A principle disadvantage of the method described above is that the insole is filled with premeasured quantities of materials, notwithstanding the fact that every foot has a different shape and thus requires an insole of different shape and containing a different volume of such materials. If the premeasured quantity of materials does not fill the space between the shoe and the foot, the insole does not conform to the foot. Moreover, if the premeasured quantity of materials exceeds the space between the shoe and the foot, the insole will become too thick and create excessive pressure on the foot.
Tansill suggests a solution for the above problem in U.S. Pat. No. 4,385,024. In that patent, he describes a method for measuring the amount of elastomeric material for each individual foot, employing special equipment. This method is cumbersome, expensive and time consuming.
In U.S. Pat. No. 4,272,898, Tansill describes a shoe insert comprising a mass of fibers which are coated with a curable resin and other fibers which are hollow and contain a curing agent. This proposal is relatively complicated and expensive for the following reasons:
1. It is difficult to fabricate the insert structure to have the hollow fibers uniformly dispersed among the coated fibers.
2. The material of the hollow fibers is required to serve initially as a barrier separating the curing agent and its vapors from the curable material and its vapors and later to be rendered frangible such that it allows a uniform release of the curing agent on the coated fibers. Tansill does not suggest suitable materials for making the hollow fibers.
3. The proposed technique of rendering the hollow fibers frangible requires special equipment and treatments.
Applicant's U.S. Pat. No. 4,716,662 describes a simple and fast method for production of shoe insoles which permits the precise control of the insole being produced. U.S. Pat. No. 4,716,662 teaches a method for casting in situ an insole on a foot comprising the steps of preparing a mold, defining an open top recess, providing a casting material in the recess, placing the foot inside the recess at a predetermined angle and position relative to the mold, engaging the casting material by the plantar surface of the foot, permitting the casting material to conform to the shape of the plantar surface of the foot, and permitting the casting material to harden.